Heat curable resins develop high heat resistance and dimensional accuracy by virtue of their crosslinked structure and thus have been expensively used in fields where high reliability is required, for example, in electronic components. There is an ever-increasing recent tendency toward an increase in density, for example, in printed wiring boards prepared using heat curable resins. For example, high adhesion to copper foils for micro wiring formation and machinability in hole making by drilling or punching are required of copper-clad laminated sheets. Further, for electronic components, mounting by lead-free solder and imparting flame retardance without use of halogens have become required from the viewpoint of recent environmental problems, and higher heat resistance and flame retardance than those of conventional heat curable resins are required of heat curable resins for copper-clad laminated sheets. Furthermore, from the viewpoint of improving safety of products and work environments, heat curable resin compositions that are composed of low-toxic ingredients only and do not evolve toxic gas and the like have been desired.
Phosphorus compounds have been proposed as halogen-free flame retardants as an alternative to bromine-containing flame retardants. Phosphorus compounds used in flame retardation include phosphoric esters such as triphenyl phosphate and cresyl diphenyl phosphate. Phosphoric esters have poor resistance to alkalis and thus pose a problem that, in producing printed boards using phosphoric ester-containing epoxy resins and the like, desmear treatment or roughening process disadvantageously causes decomposition of phosphoric acid compounds, disadvantageously leading to elution of material ingredients or an increase in water absorption of the formed printed boards (patent documents 1 to 3). Further, plasticity of these phosphoric acid compounds leads to disadvantageous phenomena such as lowered glass transition points of the resins or lowered breaking strength and fracture elongation.
In order to solve the above problems, incorporation of a phosphorus compound in an epoxy resin skeleton has been proposed (patent document 4), and it is considered that this method can reduce the problem of elution of the phosphorus compound into the treatment liquid in the desmear treatment or the roughening process and the problem of lowering in glass transition point of the resins.
Epoxy or phenolic resins with a phosphorus compound incorporated in a skeleton thereof, however, are disadvantageous in that they are expensive and, further, a large amount of phosphorus compounds should be incorporated in the resins to provide satisfactory flame retardance, leading to a deterioration in various properties of resin compositions. Furthermore, phosphorus compounds, when burned, may disadvantageously evolve toxic compounds such as phosphine.
Metal hydrates are known as flame retardants other than phosphorus compounds. For example, aluminum hydroxide is known as a flame retardant that, when heated, causes a reaction that releases water of crystallization (patent document 5). The incorporation of aluminum hydroxide into resins, when the amount of gibbsite that is a general structure of aluminum hydroxide is large, sometimes leads to lowered heat resistance of resins due to an influence of water of crystallization that is released upon heating. Further, amino triazine skeleton-containing phenolic resins that are nitrogen-containing resins have also been proposed as other flame retardants (see patent document 6). When the amount of these resins incorporated is large, heat resistance is sometime lowered due to the evolution of decomposition gas upon heating.
When the above metal hydrates that are flame retardants other than the bromine-containing flame retardants or phosphorus compounds are used, the content of the inorganic filler in the resin composition is so high that the resultant resin is hard and brittle. As a result, the abrasion speed of drill bits is high, and, hence, for example, the frequency of replacement of drill bits is increased due to breakage of drill bits or lowered accuracy of hole positions, that is, disadvantageously, the drilling workability is significantly lowered.